The United States has little doubt the Syrian government used chemical weapons against civilians last week, and any decision to open the site to UN inspectors was "too late to be credible," a senior US official has said.

"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts gathered by open sources, the US intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident," the official, who spoke on condition of anonymity, told Reuters.

He made clear the Syrian government's agreement to let United Nations inspectors visit the site of an alleged chemical weapons attack was inadequate.

"At this juncture, any belated decision by the regime to grant access to the UN team would be considered too late to be credible.

"[This is] including because the evidence available has been significantly corrupted as a result of the regime's persistent shelling and other intentional actions over the last five days," the official said.

The Syrian Foreign Ministry said earlier today it has agreed to allow UN inspectors access to sites in suburbs of Damascus where alleged chemical attacks occurred last week.

Syria denies using chemical weapons and claims to have found chemicals in tunnels used by rebel forces near to the attack site in a Damascus suburb.

Syria's principal ally Russia has suggested that forces opposed to the Assad regime may have used chemical weapons in an effort to bring western military intervention.

The US official said the administration had seen reports that Syria would provide access on Monday, but said that if the government had nothing to hide, it would have allowed investigators to visit the site five days ago.

President Barack Obama is evaluating how to respond to the incident, the official said.

"We are continuing to assess the facts so the president can make an informed decision about how to respond to this indiscriminate use of chemical weapons."

Senior US lawmakers called on Sunday for limited US military action in response to the alleged chemical weapons attack.

"I certainly would do cruise missile strikes," said Eliot Engel, the ranking Democrat on the House Foreign Affairs Committee.

Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee, told Fox News he thought Mr Obama would "respond in a surgical way."

"I hope the president as soon as we get back to Washington will ask for authorization from Congress to do something in a very surgical and proportional way," he said.

Two other Republican senators, John McCain and Lindsey Graham, issued a statement calling for "stand-off" strikes, such as by cruise missiles, to degrade the government's air power and help establish "safe areas" on the ground.